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Images of Diatom Structure. 

 By W. Balfour Stokes. 



{Bead Ajnil ISith, 1901.) 



The advantage of using a wide solid axial cone of illumination 

 lies mainly in the blotting out of those perplexing and numerous 

 interference effects so apparent when a narrow or oblique beam 

 of light is employed. 



With a wide cone, therefore, difficulties of interpretation are 

 greatly reduced ; but with many objects we may still have two 

 images, from which we must select the true one. It is easier, 

 however, to select from two than from a dozen. 



A simple diatomic structure, for instance, if examined by 

 means of a narrow or oblique beam of light, will present, at 

 different focal positions, many images or effects, not one of which 

 can be called trustworthy ; but with a wide solid axial cone 

 we obtain two images only, totally opposite in appearance and 

 at different focal positions, but both of which are conceivably 

 trustworthy representations of the only possible reality. 



These two last images I will describe as follows : 



The upper image is usually known as the '' white dot " image, 

 diatom perforations appearing as white dots. 



The lower image is known as the " black dot " image, diatom 

 perforations appearing as black dots. 



The question is : Which is the true image ? A diversity of 

 opinion seems to exist. 



Most microscopists would, I think, select the " white dot " 

 image, but Mr. Nelson prefers the "black dot," while Mr. Comber 

 suggests that the true image should lie between the two. 



The " white dot " image would be preferred by many because, 



JouRN. (^. M. C, Series II.— No. 49. 8 



